The present invention relates generally to a self locking adjustment mechanism and more particularly, to a mechanism that will enable the backrest of a chair to be repositioned and retained in a new position relative to the chair seat in a simple and reliable manner without the need for tools of any type.
U.S. Pat. No. 4,749,230 discloses a height adjusting device for a chair backrest that allows the backrest to be raised and locked in a selected position by merely pulling up on the backrest and releasing it at the desired height. The mechanism for accomplishing this is shown in FIG. 1A which is designated "PRIOR ART". This mechanism is very simple and includes two plates that slide relative to each other and a lock pin 3. The lock pin 3 is free to move within a slotted track 4 formed in a first stationary plate 1 that is carried by the chair support or seat. As the lock pin moves in the track 4, formed in the first plate 1, it engages cam surfaces, for example 5, formed in the second plate 2. The second plate 2 is carried by the backrest and this assembly moves vertically as the backrest is adjusted. The cam surfaces in the second plate cause the lock pin 3 to move from a first locked position 6 to a release position, as the backrest is raised, and then permits the lock pin to return to a second locked position 7. At the second locked position 7 the backrest has been elevated relative to the chair seat and its position at the first locked position 6. The adjusting process can be repeated for however many locked positions have been designed into the system. The mechanism is very easy to operate and the person doing the adjusting need only to lift the backrest and then release it for the backrest to become locked in the next elevated position. The backrest can be sequenced from its lowest location 6 to its highest location 8 and then locked in that position. When the backrest is raised from its lowest locked position 6 the lock pin encounters the cam surface 5 on the second plate 2 that guides the pin and causes it to roll up an incline 11 in the slotted track to a position where it clears the cam surface 5 and is then free to roll back down the incline 11 under the influence of gravity. When the lock pin reaches the bottom of the incline 11 the backrest is at its next adjusted position and is locked at that position. This process can be repeated to raise the backrest step by step to its highest locked position. With the backrest locked at its highest locked position 8, if the backrest is raised the lock pin 3 encounters a cam surface 9 on the second plate 2 that causes the lock pin to move further in the slotted track 4 than it moved when raised from the lower locked positions. This locates the lock pin in a depression or valley 12 in the slotted track. If the backrest is released, the lock pin will not slide down the incline 11 but will remain in the depression or valley 12. When the lock pin is so located there is nothing to prevent the backrest from free falling to its lowest locked position. The person doing the adjusting is accustomed to lifting the backrest, releasing it and having the backrest fall into its next adjusted position. When the person attempts the same procedure from the highest locked position, the backrest free falls to the lowest locked position. The person doing the adjusting feels that control of the adjusting mechanism has been lost and the mechanism is not operating properly. Thus, adjusting the backrest from its highest locked position can be annoying and causes the person doing the adjusting to question the proper operation of the mechanism.
For the foregoing reasons, there is a need for an adjustable mechanism for a backrest that can be positively and reliably moved from its highest locked position to its lowest position with the person doing the adjusting feeling in complete control of the adjusting mechanism and procedure.